Hi Hector,
If you have the option, I would go for the larger one.
I'm lucky to have both a 6 x 8 and an 8 x 10 (the latter under the overhang of an oak tree) but the larger one, in my experience seems to give a better return, pro rata growing space, than the smaller one (which I now treat more as a potting shed and a drying area).
Linda